LM25011MY-EVAL/NOPB

User's Guide SNVA396B – April 2009 – Revised April 2013 AN-1965 LM25011 Evaluation Board 1 Introduction The LM25011EVAL evaluation board provides the design engineer with a fully functional buck regulator, employing the constant on-time (COT) operating principle. This evaluation board provides a 5V output over an input range of 8V to 42V. The circuit delivers load currents to 1.5A, with current limit set at a nominal 1.75A. The board’s specification are: • Input Voltage: 8V to 42V • Output Voltage: 5.02V • Maximum load current: 1.5A • Minimum load current: 0A • Current Limit: ≊1.75A • Measured Efficiency: 94.1% (VIN = 8V, IOUT = 300 mA) • Nominal Switching Frequency: 750 kHz • Size: 2.6 in. x 1.6 in. TP4 SWITCH NODE J3 TP1 TP2 TP5 OUTPUT GND OUT R6 D1 R3 C3 R2 TP3 J1 J2 C4 R4 IN C1 C2 U1 R1 L1 R5 C6 GND J5 J4 P/N 551600234-001 REV D C 2009 LM25011 EVALUATION BOARD S/N Figure 1. Evaluation Board - Top Side All trademarks are the property of their respective owners. SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback AN-1965 LM25011 Evaluation Board Copyright © 2009–2013, Texas Instruments Incorporated 1 Theory of Operation 2 www.ti.com Theory of Operation Refer to the evaluation board schematic in Figure 4, which contains a simplified block diagram of the LM25011. When the circuit is in regulation, the buck switch is on each cycle for a time determined by R1 and VIN according to the equation: tON = 4.1 x 10 -11 x (R1 + 500:) VIN - 0.6V + 15 ns (1) The on-time of this evaluation board ranges from ≊893 ns at VIN = 8V, to ≊172 ns at VIN = 42V. The ontime varies inversely with VIN to maintain a nearly constant switching frequency. At the end of each ontime the Minimum Off-Timer ensures the buck switch is off for at least 150 ns. In normal operation, the offtime is much longer. During the off-time, the load current is supplied by the output capacitor (C6). When the output voltage falls sufficiently that the voltage at FB is below 2.51V, the regulation comparator initiates a new on-time period. The current limit threshold, is ≊1.75A. Refer to the LM25011/11Q/11A/11AQ 42V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit (SNVS617) data sheet for a more detailed block diagram, and a complete description of the various functional blocks. 3 Board Layout and Probing The pictorial in Figure 1 shows the placement of the circuit components. The following should be kept in mind when the board is powered: • When operating at high input voltage and high load current, forced air flow may be necessary. • The LM25011, and diode D1 may be hot to the touch when operating at high input voltage and high load current. • Use CAUTION when probing the circuit at high input voltages to prevent injury, as well as possible damage to the circuit. • At maximum load current the wire size and length used to connect the load becomes important. Ensure there is not a significant drop in the wires between this evaluation board and the load. 4 Board Connection/Start-up The input connections are made to the J1 connector. The load is connected to the OUT and GND terminals (J2 through J5). Ensure the wires are adequately sized for the intended load current. Before start-up a voltmeter should be connected to the input terminals, and to the output terminals (J2 through J5). The load current should be monitored with an ammeter or a current probe. It is recommended that the input voltage be increased gradually to 8V, at which time the output voltage should be 5V. If the output voltage is correct with 8V at VIN, then increase the input voltage as desired and proceed with evaluating the circuit. DO NOT EXCEED 45V AT VIN. 5 Current Limit Current limit detection occurs during the off-time by monitoring the voltage across the external current sense resistor R6. Referring toFigure 4, during the off-time the recirculating current flows through the inductor, through the load, through the sense resistor, and through D1 to the inductor. If the voltage across the sense resistor exceeds the threshold the current limit comparator output switches to delay the start of the next on-time period. The next on-time starts when the recirculating current decreases such that the voltage across R6 reduces to the threshold and the voltage at FB is below 2.51V. The operating frequency is typically lower due to longer-than-normal off-times. When current limit is detected, the on-time is reduced by ≊40% if the voltage at the FB pin is below its threshold when the voltage across R6 reduces to its threshold (VOUT is low due to current limiting). The current limit threshold (the valley of the inductor’s current waveform) in this evaluation board is set at 1.73A by using a 75 mohm sense resistor. The load current, at current limit detection, is that threshold plus one half the inductor’s ripple current, which ranges from 177 mAp-p (at VIN = 8V) to 424 mAp-p (at VIN = 42V). See Figure 2. The current limit threshold can be changed by replacing the sense resistor (R6) using the equation: R6 = 2 130 mV ILIM (2) AN-1965 LM25011 Evaluation Board SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Ripple Requirements www.ti.com where ILIM is the desired current limit threshold. The minimum and maximum values listed in the datasheet for the current limit threshold voltage (VILIM) should be taken into account to ensure current limit detection does not occur at less than the maximum normal load current. The maximum normal load current must not exceed 2A. If the sense resistor value is changed, check that there is sufficient ripple voltage across it, as described in the Section 6 section. Load Current Current Limit Threshold Inductor Current Voltage at the FB Pin 2.51V Normal Operation Load Current Increases Current Limited Figure 2. Normal and Current Limit Operation 6 Ripple Requirements The LM25011 requires a minimum of 10 mVp-p ripple voltage at the CS pin. That ripple voltage is generated by the decreasing recirculating current (the inductor’s ripple current) through R6 during the offtime. SeeFigure 3. Inductor Current 'I 0V Voltage at CS VRIPPLE t OFF t ON Figure 3. Ripple Voltage The ripple voltage is equal to: VRIPPLE = ΔI x R6 (3) where ΔI is the inductor current ripple amplitude, and R6 is the current sense resistor at the CS pin. In this evaluation board the inductor’s minimum ripple current is 177 mAp-p and R6 is 75 mohms, resulting in 13.3 mV for VRIPPLE. If the sense resistor value is changed in order to obtain a new current limit threshold, check that sufficient ripple voltage exists at the CS pin using the above equation. If the calculation results in less than 10 mV, the inductor value must be reduced, or the switching frequency reduced (by increasing R1), in order to increase the ripple current (ΔI). The inductor’s ripple current amplitude can be calculated from the following equation: 'I = (VIN ± VOUT) x tON L1 (4) SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback AN-1965 LM25011 Evaluation Board Copyright © 2009–2013, Texas Instruments Incorporated 3 Power Good Output www.ti.com where tON is the on-time, and L1 is the inductor value. The minimum ripple current amplitude occurs at the minimum input voltage. Alternately, the ripple current can be viewed on a scope by replacing R5 with a wire loop suitable for a scope’s current probe. 7 Power Good Output The Power Good output (PGD pin) indicates when the voltage at the FB pin is close to the internal 2.51V reference voltage. The rising threshold at the FB pin for the PGD output to switch high is 95% of the internal reference. The falling threshold for the PGD output to switch low is approximately 3.3% below the rising threshold. See Figure 14. To use the PGD output on this evaluation board an external pull-up voltage, not exceeding 7V, must be applied to TP1. The Power Good status is then available at TP2. A 10 kΩ pull-up resistor (R4) is provided on the board. 8 Soft-Start The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing startup stresses and current surges. Upon turn-on, when VIN reaches its under-voltage lock-out threshold an internal 10 µA current source charges the external capacitor at the SS pin to 2.51V. The ramping voltage at SS ramps the non-inverting input of the regulation comparator, and the output voltage, in a controlled manner. See Figure 12 and Figure 13. For proper operation, the soft-start capacitor should be no smaller than 1000 pF. On this evaluation board the soft-start time is ≊5 ms, set by C3. To change the soft-start time replace C3, using the following equation: C3 = tSS x (10 PA) 2.51V = tSS x 3.98 x 10-6 (5) An internal switch grounds the SS pin if the input voltage at VIN is below its under-voltage lock-out threshold or if the Thermal Shutdown activates. 9 Shutdown Function The LM25011 can be remotely shutdown by grounding the SS pin, accessible at TP3 on this evaluation board. Releasing the pin allows normal operation to resume. 10 Tracking Function The LM25011 can be employed as a tracking regulator by applying the controlling voltage to the SS pin, accessible on this evaluation board at TP3. The regulator’s output voltage tracks the applied voltage, gained up by the ratio of the feedback resistors. The applied voltage at the SS pin must be within the range of 0.5V to 2.6V. The absolute maximum rating for the SS pin is 3.0V. The tracking voltage applied to the SS pin must be current limited to a maximum of 1 mA. 11 Monitor The Inductor Current The inductor’s current can be monitored or viewed on a scope with a current probe. Remove R5, and install an appropriate current loop across the two large pads where R5 was located. In this way the inductor’s ripple current and peak current can be accurately determined. 12 Scope Probe Adapters Scope probe adapters are provided on this evaluation board for monitoring the waveform at the SW pin, and at the circuit’s output (VOUT), without using the probe’s ground lead which can pick up noise from the switching waveforms. The probe adapters are suitable for Tektronix P6137 or similar probes, with a 0.135” diameter. 4 AN-1965 LM25011 Evaluation Board SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Scope Probe Adapters www.ti.com 8V to 42V Input VIN C2 C1 10 PF R1 158 k: GND BST VIN L1 15 P+ C4 LM25011 0.1 PF R5 0: SW 0.1 PF RT SW D1 J1 J2 CS TP1 10 k: R4 TP2 CSG PGD TP3 Shutdown C3 0.022 PF SS SGND R6 75 m: R2 4.99 k: C6 VOUT (5V) 22 PF GND J5 FB R3 4.99 k: Figure 4. Complete Evaluation Board Schematic Table 1. Bill of Materials Item Description Mfg., Part Number Package Value C1 Ceramic Capacitor TDK C5750X7R1H106K 2220 10 µF, 50V C2 Ceramic Capacitor TDK C1608X7R1H104K 0603 0.1 µF, 50V C3 Ceramic Capacitor TDK C1608X7R1H223K 0603 0.022 µF, 50V C4 Ceramic Capacitor TDK C1608X7R1H104K 0603 0.1 µF, 50V C6 Ceramic Capacitor TDK C3225X7R1C226K 1210 22 µF, 16V D1 Schottky Diode Central Semi CMSH3-60M SMB 60V, 3A 15 µH, 2A L1 Power Inductor TDK SLF10145T-150M2R2 10 mm x 10 mm R1 Resistor Vishay CRCW0603158KFKTA 0603 158 kΩ R2 Resistor Vishay CRCW06034K99FKTA 0603 4.99 kΩ R3 Resistor Vishay CRCW06034K99FKTA 0603 4.99 kΩ R4 Resistor Vishay CRCW060310K0FKTA 0603 10 kΩ R5 Resistor Vishay CRCW08050000Z0 0805 0Ω Jumper R6 Resistor Rohm MCR18EZHFSR075 or Panasonic ERJ-LO8UF75MV 1206 75 mohm U1 Switching Regulator Texas Instruments LM25011 VSSOP SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback AN-1965 LM25011 Evaluation Board Copyright © 2009–2013, Texas Instruments Incorporated 5 Circuit Performance 13 www.ti.com Circuit Performance Figure 5. Efficiency vs Load Current Figure 6. Efficiency vs Input Voltage Figure 7. Switching Frequency vs. Input Voltage 6 AN-1965 LM25011 Evaluation Board SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Circuit Performance www.ti.com Figure 8. Line Regulation Figure 9. Load Regulation SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback AN-1965 LM25011 Evaluation Board Copyright © 2009–2013, Texas Instruments Incorporated 7 Typical Waveforms 14 www.ti.com Typical Waveforms Trace 3 = VOUT (AC Coupled) Trace 4 = Inductor Current Trace 1 = SW Node Vin = 12V, Iout = 500 mA Figure 10. Continuous Conduction Mode Trace 3 = VOUT (AC Coupled) Trace 4 = inductor Current Trace 1 = SW Node Vin = 12V, Iout = 10 mA Figure 11. Discontinuous Conduction Mode 8 AN-1965 LM25011 Evaluation Board SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated Typical Waveforms www.ti.com Trace 3 = VOUT Trace 4 = Inductor Current Trace 1 = SW Node Vin = 12V, Iout = 500 mA Figure 12. Startup Waveforms with 500 mA Load Trace 3 = VOUT Trace 4 = Inductor Current Trace 1 = SW Node Vin = 12V, Iout = 0 mA Figure 13. Startup Waveforms with No Load SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback AN-1965 LM25011 Evaluation Board Copyright © 2009–2013, Texas Instruments Incorporated 9 PC Board Layout www.ti.com Trace 3 = VOUT Trace 2 = PGD Output Trace 1 = SW Node Vin = 12V, C3 = 0.022 µF Figure 14. Startup Waveforms Showing PGD Ouput 15 PC Board Layout Figure 15. Board Silkscreen 10 AN-1965 LM25011 Evaluation Board SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback Copyright © 2009–2013, Texas Instruments Incorporated PC Board Layout www.ti.com Figure 16. Board Top Layer Figure 17. Board Second Layer (Viewed from Top) SNVA396B – April 2009 – Revised April 2013 Submit Documentation Feedback AN-1965 LM25011 Evaluation Board Copyright © 2009–2013, Texas Instruments Incorporated 11 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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